Accepting that 5E D&D is inevitable - and ignoring the debate about when it will come about - I have a concerns about the transition period that I thought I would raise.
WotC is ultimately a unit of a large American corporation, and Corporate America has it's foibles. Though there's been a string of eds of D&D, there's no guarantee Hasbro will see fit to keep green-lighting new ones. They need to see profits, and growth, for that to happen. If the market for D&D were to become 'stable' or 'mature' and stop growing, with the existing books continuing to sell, it could just sell the same ed for decades - that's how most games (boardgames and the like) work for Hasbro. You don't have a continuing evolution, just a core product and the occassional variation. D&D could be taken to that more familiar model at some point.
It could also be shelved for a decade or so to 'let demand grow.' Lots of corporations do that with their non-core products, often to the baited delight and consternation of fans - the McDonald's McRib is an obvious example of the strategy. Movies going into and out of the 'Disney Vault' is another. D&D could be relegated to that status, just trotted out every 10 years or so for a new generation of kids.
Previously, when transferring to a new edition, groups were at liberty to continue with their campaigns and use materials from the old edition until they felt they were ready to port over. Given that WotC conitnue releasing material for the old edition right up to the wire this seems like a reasonable expectation.
That worked with D&D, AD&D, and 2e, since they were largely compatible. It worked for 3.0->3.5, technically, but still generated a lot of controversey. It didn't work for 3e->4e, though, the continued availability of 3.x support (via the OGL), allowed some hardcore fans to utterly reject 4e.
I'm guessing WotC won't want to make that mistake again.
My concern is that, with 4E being very reliant on the online builder, compendium, etc, groups will find it very difficult - if not impossible - to continue with their 4E campaigns if/when WotC suddenly ceases to support them. Players may no longer have access to the characters, or be able to level them up effectively.
Your concern may be well founded. WotC won't want 4e grognards to badmouth and cut into 5e sales the way 3.5/Pathfinder did to 4e. Before there's a 5e - maybe a decade or more before (see above) - the plug will be well and truely pulled on 4e. It'll go out of print, it will not be liscenced out to third parties to support, and the digital tools will be taken down.